The present invention relates to dielectric layers for discharge lamps which are operated by means of dielectric barrier discharge, to a process for producing layers of this type and to a discharge lamp having at least one of these dielectric layers.
In lamps of this type, either the electrodes of one polarity or all the electrodes, i.e. of both types of polarity, are separated from the discharge by means of a dielectric layer (xe2x80x9cone-sided or two-sided dielectric barrier dischargexe2x80x9d). Electrodes of this type are also referred to below as xe2x80x9cdielectric electrodesxe2x80x9d for short. A dielectric layer of this type is also referred to as a dielectric xe2x80x9cbarrierxe2x80x9d or xe2x80x9cbarrier layerxe2x80x9d, and the discharge generated with an arrangement of this type is known as xe2x80x9cbarrier dischargexe2x80x9d (dielectric barrier discharge, e.g. EP-A-0 324 953, page 4).
Dielectric electrodes are produced firstly by the electrodes being arranged outside the discharge vessel, e.g. on the outer wall, for example in the form of thin, metal strips which are parallel to one another and are of alternating polarity. Discharge lamps of this type are known, for example, from WO 94/23442 (FIGS. 5a, 5b) and WO 97/04625 (FIGS. 1a, 1b).
To protect against contact and/or external influences and to avoid sliding discharges between the electrodes of different polarity, the electrode strips may advantageously be covered with a thin dielectric layer, for example with a layer of glass.
Secondly, dielectric electrodes are produced by electrodes which are arranged inside the discharge vessel and are completely covered by a dielectric layer. In so-called flat reflectors, the dielectric electrodes are typically produced in the form of thin, metal strips which are arranged on the inner wall of the discharge vessel and, in addition, either individuallyxe2x80x94by means of thin, dielectric stripsxe2x80x94or togetherxe2x80x94by means of a single, cohesive dielectric layerxe2x80x94are completely covered with respect to the interior of the discharge vessel. Discharge lamps of this type are known, for example, from EP 0 363 832 (FIG. 3) and German patent application P 197 11 892.5 (FIGS. 3a, 3b).
For the sake of simplicity, the terms xe2x80x9cdielectric barrier layers or dielectric protective layersxe2x80x9d are combined below under the term xe2x80x9cdielectric layersxe2x80x9d.
In the present context and in the text which follows, the term xe2x80x9cdischarge lampxe2x80x9d is intended to mean radiation sources which emit light, i.e. visible electromagnetic radiation, and also ultraviolet (UV) and vacuum ultraviolet (VUV) radiation.
One possible option for covering thin, strip-like electrodes with the dielectric layers described in the introduction consists in fusing a suitably sized glass film, if appropriate with the aid of an intermediate layer of soldering glass, onto the electrode strips in question.
The drawbacks of this option are firstly the relatively high costs of suitable thin glass films, and also their high sensitivity to fracture. These drawbacks have hitherto stood in the way of automated, inexpensive production.
In principle, said layers can be applied more easily and at lower cost by means of the screen-printing technique. For this purpose, glass powder (glass frit)xe2x80x94the screen-printing pastexe2x80x94which is dispersed in a suitable organic solventxe2x80x94the so-called screen-printing mediumxe2x80x94is applied to the electrodes and to that surface of the discharge vessel which surrounds the electrodes with the aid of a so-called squeegee and a resilient screen. The screen is initially arranged at a certain distance from the surface. During the application, the squeegee passes over the screen, so that this screen, together with the printing paste, is pressed onto the surface. In the process, the squeegee fills the meshes in the screen with the printing paste, the squeegee simultaneously wiping the excess printing paste off the screen. After the squeegee has passed over the coated meshes, the corresponding meshes are lifted off the surface again, and the applied printing paste remains on the surface. After drying, the layer applied is fused, so that a hermetically sealed surface which is as far as possible planar and free of pores is formed. This is important since the thickness of the layer is a parameter which has a direct effect on the dielectric discharge, on the one hand, and the high-voltage contact protection, on the other hand.
However, a drawback is that the surface stress of the fused layer of soldering glass prevents complete wetting of the electrodes. Rather, it has been found that the fused soldering glass draws back from the metallic electrodes over a large area.
The object of the present invention is to eliminate the above drawbacks and to provide a dielectric layer which completely covers at least a partial region of one or more electrodes and, in addition, covers at least the discharge-vessel wall which is immediately adjacent to this partial region of the electrode. A further aspect of the invention is that this layer be suitable as a dielectric barrier for a dielectric barrier discharge, in particular for a pulsed dielectric barrier discharge.
This object is achieved by the features of claim 1. Further advantageous features are given in the claims which are dependent on claim 1.
A further object is to specify a printing process for applying a dielectric layer, in which the printing paste, in the molten state, completely wets at least a partial region of metallic electrodes and also wets the discharge-vessel wall which is immediately adjacent to this partial region of the electrode, and consequently, after it has been fired, also completely covers the at least one partial region of the electrodes, including the immediately adjacent discharge-vessel wall, with a dielectric layer.
This object is achieved by the features of the process claim. Further advantageous features are given in the claims which depend on this claim.
Moreover, protection is claimed for a discharge lamp which has at least one electrode which is covered with a dielectric layer produced using the process according to the invention:
According to the invention, the dielectric layer, which is produced substantially from a powder or powder mixture of vitreous substances, additionally contains at least one additive, the melting temperature of which is higher than the melting temperature of the glass powder or the glass powder component with the highest melting temperature. Consequently, the fired layer comprises a vitreous main component in which the at least one additive is included in dispersed form, for example in the form of grains.
If Ts1 denotes the melting temperature of the glass powderxe2x80x94which is typically approximately 400 to 700xc2x0 C. xe2x80x94and Ts2 denotes the melting temperature of the additive, the following relationship applies: Ts2 greater than Ts1. It has been found that good results can be achieved with additives whose melting temperature is at least 100xc2x0 C. higher than the melting temperature of the glass powder or the glass powder component with the highest melting temperature, i.e. the following relationship applies: Ts2xe2x89xa7100xc2x0 C.+Ts1, where the values for Ts1 and Ts2 are to be given in xc2x0 C.
Suitable additives are in particular powders comprising ceramic substances and/or crystalline or amorphous metal oxides, e.g. crystalline or amorphous aluminum oxide powder with a melting temperature of more than 2000xc2x0 C. and/or quartz glass powder with a melting temperature of more than 1400xc2x0 C. The proportion by weight made up by the additive or additives is between approximately 2% and 30%, preferably between 5% and 20%. Below the lower limit, the positive effect of the at least one additive is no longer sufficient. Above the upper limit, cracks and similar mechanical disruptions to the layer occur to an unacceptable extent.
The process according to the invention for producing the abovementioned dielectric layer proposes that the abovementioned at least one additive be admixed with the printing paste containing the glass powder prior to the actual printing process, advantageously in fine-grained form. As has already been mentioned, the proportion by weight made up by the additive or additives is between approximately 2% and 30%, preferably between 5% and 20%. In this context, it is essential for the effect according to the invention that the at least one additive be specifically selected in such a manner that its melting temperature is higher than the firing temperature required for fusing the glass powder. Otherwise, the statements which have already been made in connection with the explanation of the dielectric layer apply in terms of suitable additives.
A recommended suitable printing process is the standard screen-printing process. To produce dielectric layers of relatively great thickness, further printing and melting operations are applied to the previous layer(s). Since in this case there is no longer any need to cover any free electrode surfaces, and consequently there are also no longer any wetting problems, these subsequent layers can also be produced from pure soldering glass powder, i.e. without additive(s).
The printing paste according to the invention, i.e. printing paste including additive(s), can also be used, in a simple, readily automatable and consequently inexpensive way, to apply dimensionally stable dielectric layers of any desired dimensions to metallic electrodes and the surrounding surface of the discharge vessel, specifically with a wetting behavior which is improved compared to previous pastes.